Dielectric contact etch

ABSTRACT

A method for forming a semiconductor device in a plasma processing chamber is provided. An atomic layer etch selectively etches SiO with respect to SiN and deposits a fluorinated polymer. The fluorinated polymer layer is stripped, comprising flowing a stripping gas comprising oxygen into the plasma processing chamber, forming a plasma from the stripping gas, and stopping the flow of the stripping gas. A SiN layer is selectively etched with respect to SiO and SiGe and Si.

BACKGROUND

The disclosure relates to a method of forming semiconductor devices on asemiconductor wafer. More specifically, the disclosure relates to fintransistors.

In forming semiconductor devices, FinFET fabrication and epitaxy may beused to provide fin transistors.

SUMMARY

To achieve the foregoing and in accordance with the purpose of thepresent disclosure, a method for forming a semiconductor device in aplasma processing chamber is provided. An atomic layer etch selectivelyetches SiO with respect to SiN and deposits a fluorinated polymer. Thefluorinated polymer layer is stripped, comprising flowing a strippinggas comprising oxygen into the plasma processing chamber, forming aplasma from the stripping gas, and stopping the flow of the strippinggas. A SiN layer is selectively etched with respect to SiO and SiGe andSi.

These and other features of the present disclosure will be described inmore details below in the detailed description and in conjunction withthe following figures.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings and in whichlike reference numerals refer to similar elements and in which:

FIG. 1 is a high level flow chart of an embodiment.

FIG. 2 is a more detailed flow chart of the fluorinated polymerdeposition phase.

FIG. 3 is a more detailed flow chart of the activation phase.

FIGS. 4A-F are schematic cross-sectional views of a structure processedaccording to an embodiment.

FIG. 5 is a schematic view of a plasma processing chamber that may beused in an embodiment.

FIG. 6 is a schematic view of a computer system that may be used inpracticing an embodiment.

FIG. 7 is a more detailed flow chart of the oxygen stripping process.

FIG. 8 is a more detailed flow chart of the SiN etching process.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present disclosure will now be described in detail with reference toa few preferred embodiments thereof as illustrated in the accompanyingdrawings. In the following description, numerous specific details areset forth in order to provide a thorough understanding of the presentdisclosure. It will be apparent, however, to one skilled in the art,that the present disclosure may be practiced without some or all ofthese specific details. In other instances, well known process stepsand/or structures have not been described in detail in order to notunnecessarily obscure the present disclosure.

FIG. 1 is a high level flow chart of an embodiment. In this embodiment,a substrate with a structure, such as a FinFET structure with epitaxialgrowth, is placed in a plasma processing chamber (step 104). A cyclicalatomic layer etching of SiO is provided (step 108). Each cycle of theatomic layer etching process comprises a fluorinated polymer depositionphase (step 112) and an activation phase (step 116). The unwantedfluorinated polymer is stripped using an oxygen strip (step 120). A SiNlayer is selectively etched (step 124). The substrate with the structureis removed from the plasma processing chamber (step 128). FIG. 2 is amore detailed flow chart of fluorinated polymer deposition phase (step112). A fluorinated polymer deposition gas is flowed into the plasmaprocessing chamber (step 204). The fluorinated polymer deposition gas isformed into a plasma, which deposits fluorocarbon polymer on thestructure (step 208). The flow of the fluorinated polymer deposition gasis stopped (step 212). FIG. 3 is a more detailed flow chart ofactivation phase (step 116). An activation gas is flowed into the plasmaprocessing chamber, where the activation gas comprises an inertbombardment gas (step 304). The activation gas is formed into a plasma,which activates fluorine and carbon in the fluorinated polymer, whichcauses SiO to be selectively etched with respect to SiN (step 308). Theflow of the activation gas is stopped (step 312).

EXAMPLE

In an example, a substrate with a structure is placed in a plasmaprocessing chamber (step 104). FIG. 4A is a schematic cross-sectionalview of a structure 400, with a substrate 404, under a silicon layer408, under a dielectric layer 418 with a FinFET structure 420. TheFinFET structure 420 is of either SiGe or Si or a combination thereof.In the specification and claims, SiGe or Si includes doped and undopedSiGe or Si. The top of the FinFET structure 420 has been enlarged usingepitaxy. A coated etch stop layer 424 of SiN has been conformallydeposited around the enlarged top of the FinFET structure 420. Ahardmask 428 is formed over the dielectric layer 418.

FIG. 5 is a schematic view of a plasma processing chamber that may beused in an embodiment. In one or more embodiments, the plasma processingsystem 500 comprises a gas distribution plate 506 providing a gas inletand an electrostatic chuck (ESC) 508, within a processing chamber 549,enclosed by a chamber wall 550. Within the processing chamber 549, thesubstrate 404 is positioned on top of the ESC 508. The ESC 508 mayprovide a bias from the ESC source 548. A gas source 510 is connected tothe plasma processing chamber 549 through the distribution plate 506.The gas source 510 comprises multiple gas sources. An ESC temperaturecontroller 551 is connected to the ESC 508, and provides temperaturecontrol of the ESC 508. In this example, a first connection 513 providespower to an inner heater 511 for heating an inner zone of the ESC 508,and a second connection 514 provides power to an outer heater 512 forheating an outer zone of the ESC 508. An RF source 530 provides RF powerto a lower electrode 534. An upper electrode, in this embodiment, is thegas distribution plate 506 and is electrically grounded. In a preferredembodiment, 2 MHz, 60 MHz, and 27 MHz power sources make up the RFsource 530 and the ESC source 548. In this embodiment, one generator isprovided for each frequency. In other embodiments, the generators may bein separate RF sources, or separate RF generators may be connected todifferent electrodes. For example, the upper electrode may have innerand outer electrodes connected to different RF sources. A controller 535is controllably connected to the RF source 530, the ESC source 548, anexhaust pump 520, and the etch gas source 510. An example of such aplasma processing chamber is the Exelan Flex™ etch system manufacturedby Lam Research Corporation of Fremont, Calif. The process chamber canbe a CCP (capacitive coupled plasma) reactor or an ICP (inductivecoupled plasma) reactor.

FIG. 6 is a high level block diagram showing a computer system 600,which is suitable for implementing a controller 535 used in embodiments.The computer system may have many physical forms ranging from anintegrated circuit, a printed circuit board, and a small handhelddevice, up to a huge super computer. The computer system 600 includesone or more processors 602, and further can include an electronicdisplay device 604 (for displaying graphics, text, and other data), amain memory 606 (e.g., random access memory (RAM)), storage device 608(e.g., hard disk drive), removable storage device 610 (e.g., opticaldisk drive), user interface devices 612 (e.g., keyboards, touch screens,keypads, mice or other pointing devices, etc.), and a communicationinterface 614 (e.g., wireless network interface). The communicationinterface 614 allows software and data to be transferred between thecomputer system 600 and external devices via a link. The system may alsoinclude a communications infrastructure 616 (e.g., a communications bus,cross-over bar, or network) to which the aforementioned devices/modulesare connected.

Information transferred via communications interface 614 may be in theform of signals such as electronic, electromagnetic, optical, or othersignals capable of being received by communications interface 614, via acommunication link that carries signals and may be implemented usingwire or cable, fiber optics, a phone line, a cellular phone link, aradio frequency link, and/or other communication channels. With such acommunications interface, it is contemplated that the one or moreprocessors 602 might receive information from a network, or might outputinformation to the network in the course of performing theabove-described method steps. Furthermore, method embodiments mayexecute solely upon the processors or may execute over a network such asthe Internet, in conjunction with remote processors that share a portionof the processing.

The term “non-transient computer readable medium” is used generally torefer to media such as main memory, secondary memory, removable storage,and storage devices, such as hard disks, flash memory, disk drivememory, CD-ROM, and other forms of persistent memory, and shall not beconstrued to cover transitory subject matter, such as carrier waves orsignals. Examples of computer code include machine code, such as oneproduced by a compiler, and files containing higher level code that areexecuted by a computer using an interpreter. Computer readable media mayalso be computer code transmitted by a computer data signal embodied ina carrier wave and representing a sequence of instructions that areexecutable by a processor.

After the structure 400 with the substrate 404 has been placed into theplasma processing chamber 549, cyclical atomic layer etching is provided(step 108). Each cycle of the atomic layer etching process comprises afluorinated polymer deposition phase (step 112) and an activation phase(step 116). FIG. 2 is a more detailed flow chart of fluorinated polymerdeposition phase (step 112). A fluorinated polymer deposition gas isflowed into the plasma processing chamber (step 204). In thisembodiment, the flowing the fluorinated polymer deposition gas flowsbetween 1 to 20 sccm of C₄F₆, 1 to 520 sccm O₂, 10 to 300 sccm CO, and100 to 1000 sccm Ar. The fluorinated polymer deposition gas is formedinto a plasma, which deposits fluorocarbon polymer on the structure(step 208). RF power is provided at 60 MHz and/or 27 MHz at a powerbetween 0-200 Watts is provided to transform the deposition gas into aplasma. The flow of the fluorinated polymer deposition gas is stopped(step 212) after 1 to 15 seconds. Other Fluorocarbons can be used asdepositing gases.

FIG. 4B is a schematic cross-sectional view of a structure 400 after thefluorinated polymer deposition phase is completed. A thin conformallayer or coating of fluorinated polymer 432 is deposited on thestructure 400. The coating of fluorinated polymer 432 is not drawn toscale, but is drawn thicker to better facilitate understanding.

FIG. 3 is a more detailed flow chart of activation phase (step 116). Anactivation gas is flowed into the plasma processing chamber, where theactivation gas comprises an inert bombardment gas (step 304). In thisexample, the activation gas comprises 100-1000 sccm Ar only. In thisexample, the activation gas is Hydrogen free. The activation gas isformed into a plasma, which activates fluorine and carbon in thefluorinated polymer, which causes SiO to be selectively etched withrespect to SiN, SiGe, Si, and doped Si (step 308). In this example,30-400 Watts of RF power is provided at 60 MHz and 30-300 Watts of RFpower is provided at 27 MHz. The flow of the activation gas is stopped(step 312) after a few seconds.

FIG. 4C is a schematic cross-sectional view of a structure 400 after theactivation phase is completed. The plasma generated from the inertbombardment gas activates the fluorine and carbon in the fluorinatedpolymer, which cause part of the dielectric layer 418 to be etched withrespect to SiN. A layer of the dielectric layer 418 is shown as beingetched away.

The cyclical process is repeated many times. In this example, theprocess is repeated from 5-50 times. FIG. 4D is a schematiccross-sectional view of a structure 400 after the atomic layer etch hasbeen performed for 15 cycles. The atomic layer etch selectively etchedthe SiO layers with respect to SiN, SiGe, Si, and doped Si. Afluorinated polymer layer 436 is deposited over an exposed part of theetch stop layer 424.

An oxygen strip is provided to strip the fluorinated polymer layer (step120). FIG. 7 is a more detailed flow chart of the oxygen strip in thisexample (step 120). A stripping gas comprising oxygen is flowed into theplasma processing chamber (step 704). In this example the stripping gasis 200-2000 sccm O₂. The strip may comprise other gases like CO2, CO andor noble gases. The stripping gas is formed into a plasma (step 708). Inthis example, RF power is provided at 200 Watts at 60 MHz. The flow ofthe stripping gas is stopped (step 712).

FIG. 4E is a schematic cross-sectional view of a structure 400 after theoxygen strip is completed. The fluorinated polymer layer has beenstripped, without removing the etch stop layer 424.

The SiN is selectively etched (step 124). FIG. 8 is a more detailed flowchart of selectively etching SiN (step 124). A SiN etch gas is flowedinto the plasma processing chamber (step 804). In this example, the SiNetch gas is 10-200 sccm CH₃F and 100-800 sccm H₂. The SiN etch gas isformed into a plasma (step 808). In this example, the RF power isprovided at a frequency of 60 MHz and a power between 50 and 250 W. TheRF power is pulsed at a 50% duty cycle with a frequency between 50-500Hz. The flow of the stripping gas is stopped (step 812). Otherhydrofluorocarbons and hydrocarbons can be added.

FIG. 4F is a schematic cross-sectional view of a structure 400 after theSiN of the etch stop layer 424 is selectively etched. The etch stoplayer 424 is selectively etched so that the SiO of the dielectric layer418 and the FinFET structure 420 have minimal etching.

The substrate 404 with the structure 400 is removed from the plasmaprocessing chamber (step 128). Additional steps may be performed beforethe structure 400 is removed from the plasma processing chamber.

It was found that the atomic layer etch has a SiO to SiN etchselectivity greater than 10:1. In some embodiments, the thickness of theetch stop layer 424 is less than 5 nm. Without such a high SiO to SiNetch selectivity during the etching of the SiO dielectric layer 418, theetch stop layer 424 may be etched away to expose the epitaxial part ofthe FinFET structure 420, which may damage the device and cause defects.In order to selectively etch the etch stop layer 424 with respect toSiO, SiGe, and Si, the fluorinated polymer must be sufficiently removed.It has been found that stripping the fluorinated polymer without oxygendoes not sufficiently remove the fluorinated polymer. The prior artavoided the use of oxygen in such processes, since oxygen would damagethe epitaxy part of the FinFET structure 420. Because of the highselectivity of the atomic layer etch, the etch stop layer 424 is notover etched and provides sufficient protection of the FinFET structure420 to prevent damage from oxygen. The use of oxygen in strippingsufficiently removes the fluorinated polymer, so that the SiN may behighly selectively etched. The etch selectivity of SiN to SiO is atleast 10:1 in steady state. In addition, the etch selectivity of SiN toSiGe and Si is at least 10:1 in steady state. Without a high selectivityfor etching SiN with respect to SiO, etching the SiN would further etchand change the dielectric layer 418. Without high etch selectivity foretching SiN with respect to SiGe and Si, etching the SiN could damagethe FinFET structure 420, causing device performance degradation. Inaddition, the atomic layer etch of SiO was unexpectedly found to havevarious embodiments that are able to provide small CD etch wrap around,which allows etching around a structure, and then past the structure.Etch wrap around improves the resulting contacts, since etch wrap aroundprovides a contact along the angled surface of the epitaxy portion ofthe FinFET structure 420. In various embodiments, one or more layers maybe between or over or under the various layers. In various embodiments,additional steps or phases may be added. In other embodiments, thevarious steps may be performed in different processing chambers. Forexample, the stripping may be performed in a different chamber than theatomic layer etching of SiO.

In addition to a fluorocarbon, the fluorinated polymer deposition gasmay further comprise CO, O₂, CO₂, and/or Ar. The CO provides more carbondeposition on top of areas with low aspect ratios than areas with highaspect ratios, which further slows the etching of SiN in low aspectratio regions. Preferably, the activation gas is fluorine free.Preferably, the activation gas is hydrogen free. Preferably, the biasvoltage during the fluorinated polymer deposition phase and theactivation phase has a magnitude of less than 200 volts.

The amount or thickness of the fluorinated polymer provides aself-limitation of the amount of etching during the activation phase. Inthe specification and claims, atomic layer etching is defined as aself-limited etching process, such as the self-limitation of theactivation process due to the limited presence of fluorinated polymer.

Preferably, the SiO is selectively etched with respect to SiGe and Si ata ratio of at least 10:1. More preferably, the SiO is selectively etchedwith respect to SiGe and Si at a ratio of at least 20:1. Preferably, theSiO is selectively etched with respect to SiN at a ratio of at least10:1. More preferably, the SiO is selectively etched with respect to SiNat a ratio of at least 20:1. Preferably, the SiN is selectively etchedwith respect to SiGe and Si at a ratio of at least 10:1. Morepreferably, the SiN is selectively etched with respect to SiGe and Si ata ratio of at least 20:1. Preferably, the SiN is selectively etched withrespect to SiO at a ratio of at least 10:1. More preferably, the SiN isselectively etched with respect to SiO at a ratio of at least 20:1. Invarious embodiments, the doped Si may be silicon doped with carbon,phosphorus, or boron.

Embodiments offer a way to substantially improve the performance of adevice, by tightly controlling critical dielectric etch process setpoints and parameters. In particular, dielectric contact etchrequirements lead to prolonged exposure to plasma of the parts oftransistor. This can cause irreversible damage of the source and drainepitaxial growth region (epi) of the fin transistor and hence, degradedevice performance.

Even though design of a device provides certain protection of atransistor's active areas, usually a thin silicon nitride (SiN) layerseparating dielectric (typically silicon oxide or just oxide) and activetwo-dimensional areas of a transistor, this SiN layer may be removednon-selectively to oxide on the same process step called “over-etch”(OE). If the SiN layer is too thin (<5 nm), in the prior art, it willdefinitely be removed, causing unwanted exposure to powerful OE plasmaand, hence, epi damage expressed in the physical loss of epi material inthe regions of epitaxial growth most critical in the device performance.

Various embodiments provide a way to stop on a thin (˜4 nm) SiN layerprotecting epitaxial growth, during an oxide OE recipe step, and thengently remove the SiN layer, causing minimal or no damage to underlyingepitaxial growth. To achieve this, an atomic layer etch (ALE) techniqueis used on an oxide OE step to ensure high selectivity to the SiN layer(often called “stop layer”). Once the oxide is cleared, a short stripstep is applied to remove unwanted polymer followed by very low powerselective SiN removal step (called LRM). This step exhibits excellentselectivity to both oxide still exposed in some areas and epitaxialgrowth. The epitaxial growth remains practically intact. Since the LRMstep is capable of removing SiN selective to epi, the obvious benefit isthe increase of contact area between metal of a contact and epi (goodepi wraparound). This leads to contact resistance reduction, whichfurther improves device performance.

In various embodiments, a conventional powerful non-selective oxideover-etch recipe step is replaced by a sequence of low power highselectivity steps to minimize or even eliminate a transistor's epi loss,while keeping other critical process specification parameters undercontrol. The “all-in-one” oxide and SiN removal step is replaced withthe sequence of recipe steps slated to highly increase selectivity tothe appropriate layers, aiming to minimize unwanted material loss.

In various embodiments, for oxide removal selective to underlying SiN,an ALE technique is applied, which is designed to boost oxide/SiNselectivity to 10 and above. This allows the oxide to be removed in thecontact, while a SiN stop layer remains in place to protect underlyingepi. Once the oxide is cleared with certain amount of over-etch, anoxygen strip is applied to clean up any polymer left over from ALE step,to ensure clean SiN is exposed for the subsequent dedicated SiN removalstep. The remaining SiN layer protects Si epitaxial growth fromoxidation during the strip. Upon completion of strip step, the SiNremoval step (LRM) is applied. This step is developed to remove SiNhighly selectively (selectivity >10) to both oxide (e.g. sidewalls offreshly etched contact) and epitaxial growth, where SiN is supposed tobe removed from. In other embodiments, the fluorocarbon gas may be oneor more of C₃F₆, C₄F₈, C₅F₈, or C₆F₆. In addition, preferablyfluorocarbon gas does not include hydrofluorocarbons. In otherembodiments, the SiN etching gas may use O₂ or CO₂ in place of or incombination with H₂.

While this disclosure has been described in terms of several preferredembodiments, there are alterations, modifications, permutations, andvarious substitute equivalents, which fall within the scope of thisdisclosure. It should also be noted that there are many alternative waysof implementing the methods and apparatuses of the present disclosure.It is therefore intended that the following appended claims beinterpreted as including all such alterations, modifications,permutations, and various substitute equivalents as fall within the truespirit and scope of the present disclosure.

What is claimed is:
 1. A method for forming a semiconductor device froma structure comprising a silicon oxide layer and a SiN layer and astructure of at least one of SiGe or Si in a plasma processing chamber,comprising: providing an atomic layer etch that selectively etches thesilicon oxide layer with respect to the SiN layer and deposits afluorinated polymer layer; stripping the fluorinated polymer layer,comprising: flowing a stripping gas comprising oxygen into the plasmaprocessing chamber; forming a plasma from the stripping gas; andstopping the flow of the stripping gas; and selectively etching the SiNlayer, with respect to the silicon oxide layer and the structure of atleast one of SiGe and Si after stripping the fluorinated polymer layer.2. The method, as recited in claim 1, wherein the stripping gas consistsessentially of oxygen.
 3. The method, as recited in claim 2, wherein theselective etching of the SiN layer, comprises: flowing an etch gascomprising CH₃F and at least one of H₂, O₂, or CO₂ into the plasmaprocessing chamber; and forming the etch gas into a plasma.
 4. Themethod, as recited in claim 3, wherein the providing the atomic layeretch, comprises providing a plurality of cycles, wherein each cyclecomprises: a fluorinated polymer deposition phase, comprising: flowing afluorinated polymer deposition gas comprising a fluorocarbon gas intothe plasma processing chamber; forming the fluorinated polymerdeposition gas into a plasma, which deposits a fluorocarbon polymerlayer; and stopping the flow of the fluorinated polymer deposition gasinto the plasma processing chamber; and an activation phase, comprising:flowing an activation gas comprising an inert bombardment gas into theplasma processing chamber; forming the activation gas into a plasma,wherein the inert bombardment gas activates fluorine and carbon in thefluorinated polymer layer which causes silicon oxide to be selectivelyetched with respect to SiN; and stopping the flow of the activation gasinto the plasma processing chamber.
 5. The method, as recited in claim4, wherein the activation gas is hydrogen free.
 6. The method, asrecited in claim 5, wherein the activation gas is fluorine free.
 7. Themethod, as recited in claim 4, wherein the activation gas consistsessentially of Ar.
 8. The method, as recited in claim 4, wherein thefluorinated polymer deposition phase provides a bias voltage with amagnitude of less than 200 volts and the activation phase provides abias with a magnitude of less than 200 volts.
 9. The method, as recitedin claim 4, wherein the fluorinated polymer deposition phase gas furthercomprises CO.
 10. The method, as recited in claim 9, wherein thefluorinated polymer deposition phase gas further comprises O₂ or Ar. 11.The method, as recited in claim 4, wherein the atomic layer etchselectively etches silicon oxide with respect to silicon nitride at aratio greater than 10:1.
 12. The method, as recited in claim 4, whereinthe selectively etching the SiN layer selectively etches SiN withrespect to SiGe and Si at a ratio of at least 10:1.
 13. The method, asrecited in claim 4, wherein the semiconductor device comprises a fintransistor with an epitaxial growth region protected by a layer of SiNwith a thickness of less than 5 nm.
 14. The method, as recited in claim1, wherein the selective etching of the SiN layer, comprises: flowing anetch gas comprising CH₃F and at least one of H₂, O₂, or CO₂ into theplasma processing chamber; and forming the etch gas into a plasma. 15.The method, as recited in claim 1, wherein the providing the atomiclayer etch, comprises providing a plurality of cycles, wherein eachcycle comprises: a fluorinated polymer deposition phase, comprising:flowing a fluorinated polymer deposition gas comprising a fluorocarboninto the plasma processing chamber; forming the fluorinated polymerdeposition gas into a plasma, which deposits a fluorocarbon polymerlayer; and stopping the flow of the fluorinated polymer deposition gasinto the plasma processing chamber; and an activation phase, comprising:flowing an activation gas comprising an inert bombardment gas into theplasma processing chamber; forming the activation gas into a plasma,wherein the inert bombardment gas activates fluorine and carbon in thefluorinated polymer layer which causes silicon oxide to be selectivelyetched with respect to SiN; and stopping the flow of the activation gasinto the plasma processing chamber.
 16. The method, as recited in claim15, wherein the activation gas consists essentially of Ar.
 17. Themethod, as recited in claim 1, wherein the structure of at least one ofSiGe or Si comprises SiGe.
 18. The method, as recited in claim 15,wherein the structure of at least one of SiGe or Si comprises a fintransistor with an epitaxial growth region protected by the SiN layer,wherein the SiN layer has a thickness of less than 5 nm.
 19. A methodfor forming a semiconductor device with at least one structure of SiNand at least one structure of silicon oxide and at least one structureof SiGe or Si, in a plasma processing chamber, comprising: providing anatomic layer etch that selectively etches the at least one structure ofsilicon oxide with respect to the at least one structure of SiN anddeposits a fluorinated polymer layer; stripping the fluorinated polymerlayer, comprising: flowing a stripping gas into the plasma processingchamber; forming a plasma from the stripping gas; and stopping the flowof the stripping gas; and selectively etching the at least one structureof SiN, with respect to the at least one structure of silicon oxide andthe at least one structure of SiGe or Si after stripping the fluorinatedpolymer layer.
 20. The method, as recited in claim 19, wherein theproviding the atomic layer etch, comprises providing a plurality ofcycles, wherein each cycle comprises: a fluorinated polymer depositionphase, comprising: flowing a fluorinated polymer deposition gascomprising a fluorocarbon into the plasma processing chamber; formingthe fluorinated polymer deposition gas into a plasma, which deposits afluorocarbon polymer layer; and stopping the flow of the fluorinatedpolymer deposition gas into the plasma processing chamber; and anactivation phase, comprising: flowing an activation gas comprising aninert bombardment gas into the plasma processing chamber; forming theactivation gas into a plasma, wherein the inert bombardment gasactivates fluorine and carbon in the fluorinated polymer layer whichcauses silicon oxide to be selectively etched with respect to SiN; andstopping the flow of the activation gas into the plasma processingchamber.